1. Field of the Invention
The invention relates generally to thin-walled, impact-absorbing compressible members. More specifically, the invention relates to an air venting, impact-absorbing compressible member, preferably fabricated from thermoplastic elastomer (TPE) material, that can be used in the construction of a wide variety of shock-absorbing and/or impact protective devices, including, without limitation, protective headgear, protective pads for other parts of the body, protective padding for sports arenas such as hockey rink boards and the like, and impact-absorbing devices for vehicles such as bumpers, dashboards and the like.
2. Background Information
Concussions, also called mild traumatic brain injury, are a common, serious problem in sports known to have detrimental effects on people in the short and long term. With respect to athletes, a concussion is a temporary and reversible neurological impairment, with or without loss of consciousness. Another definition of a concussion is a traumatically induced alteration of brain function manifested by 1) an alteration of awareness or consciousness, and 2) signs and symptoms commonly associated with post-concussion syndrome, such as persistent headaches, loss of balance, and memory disturbances, to list but a few. Some athletes have had their careers abbreviated because of concussions, in particular because those who have sustained multiple concussions show a greater proclivity to further concussions and increasingly severe symptoms. Although concussions are prevalent among athletes, the study of concussions is difficult, treatment options are virtually non-existent, and “return-to-play” guidelines are speculative. Accordingly, the best current solution to concussions is prevention and minimization.
Concussion results from a force being applied to the brain, usually the result of a direct blow to the head, which results in shearing force to the brain tissue, and a subsequent deleterious neurometabolic and neurophysiologic cascade. There are two primary types of forces experienced by the brain in an impact to the head, linear acceleration and rotational acceleration. Both types of acceleration are believed to be important in causing concussions. Decreasing the magnitude of acceleration thus decreases the force applied to the brain, and consequently reduces the risk or severity of a concussion.
Protective headgear is well known to help protect wearers from head injury by decreasing the magnitude of acceleration (or deceleration) experienced by the wearers. Currently marketed helmets primarily address linear forces, but generally do not diminish the rotational forces experienced by the brain. Helmets fall generally into two categories: single impact helmets and multiple-impact helmets. Single-impact helmets undergo permanent deformation under impact, whereas multiple-impact helmets are capable of sustaining multiple blows. Applications of single-impact helmets include, for example, bicycling and motorcycling. Participants of contact sports, such as hockey and football, use multiple-impact helmets. Both categories of helmets have similar construction. A semi-rigid outer shell distributes the force of impact over a wide area and a compressible foam inner layer reduces the force upon the wearer's head.
The inner layer of single-impact helmets are typically constructed of fused expanded polystyrene (EPS), a polymer impregnated with a foaming agent. EPS reduces the amount of energy that reaches the head by permanently deforming under the force of impact. To be effective against the impact, the inner layer must be sufficiently thick not to crush entirely throughout its thickness. A thick inner layer, however, requires a corresponding increase in the size of the outer shell, which increases the size and bulkiness of the helmet.
Inner layers designed for multiple-impact helmets absorb energy through elastic and viscoelastic deformation. To absorb multiple successive hits, these helmets is need to rebound quickly to return to their original shape. Materials that rebound too quickly, however, permit some of the kinetic energy of the impact to transfer to the wearer's head. Examples of materials with positive rebound properties, also called elastic memory, include foamed polyurethane, expanded polypropylene, expanded polyethylene, and foamed vinylnitrile. Although some of these materials have desirable rebound qualities, an inner layer constructed therefrom must be sufficiently thick to prevent forceful impacts from penetrating its entire thickness. The drawback of a thick foam layer, as noted above, is the resulting bulkiness of the helmet. Moreover, the energy absorbing properties of such materials tend to diminish with increasing temperatures, whereas the positive rebound properties diminish with decreasing temperatures.
Regardless of the particular material involved, the material properties and densities of foam inner layers in helmets have historically been selected to optimally absorb energy for impacts that are considered severe for the particular sport or activity in which the helmets are to be used. Foams are thus relatively ineffective in absorbing impact energies below the severe level. Industry safety standards currently test and certify helmet designs based on their ability to absorb high energy impacts to ensure that helmets protect wearers against severe head injuries, such as skull fractures. Recent evidence has shown that lower energy impacts result in less severe yet still damaging head injuries, typically concussions. Current laboratory certification tests are pass/fail tests, and are not designed to test for prevention of concussions.
As such, testing of helmets to protect against concussions is being developed outside the realm of existing industry standards as the industry attempts to determine if helmets can be designed that provide universal protection against both mild and severe impacts. Several manufacturers are experimenting with various permutations of laminated foams and newer materials to broaden the range of impact energies over which the materials provide effective energy absorption. While some progress is being made, it is limited. This is due at least in part to the fact that foam materials are inherently limited in their ability to absorb energy because of their tendency to “bottom out” when compressed. Specifically, foams can be compressed downwardly only about seventy percent (70%) from their uncompressed thicknesses before they become so dense and stiff that they no longer effectively absorb impact energy. This factor is referred to as the “ride-down” point of the foam. When compressed to the is maximum “ride-down” point, a foam in a helmet is said to have “bottomed out”, and acts essentially as a rigid layer that transfers impact energy with little or no absorption directly to the wearer's head.
There is thus a need in the industry for an improved helmet construction that can reduce the risk and severity of head injuries, including concussions, over a wide range of impact energies, without the aforementioned disadvantages of current helmet designs. There exists a similar need for structures that have improved impact-absorbing properties for use in a variety of other applications.